sphere and disk area lights mostly ready (I think :S).

plane.cpp

@@ -19,3 +19,11 @@ bool Plane::intersect(Ray & r, float & t) const {
 vec3 Plane::normal_at_int(Ray & r, float & t) const {
   return vec3(m_normal);
 }
+
+vec3 Plane::sample_at_surface() const {
+  return vec3(0.0f);
+}
+
+void Plane::calculate_inv_area() {
+  m_inv_area = 0.0f;
+}